Commonly-owned U.S. Pat. Nos. 5,475,355 and 5,495,222, whose contents are herein incorporated by reference, describe, among other things, a generally C-shaped magnetic structure for generating uniform magnetic fields in a region of interest, for example, for use in magnetic resonance imaging (MRI) by means of a relatively open magnetic structure allowing surgical intervention during an MRI procedure. Among the features described in those two patents are various schemes for compensating for field distortions introduced by, among other things, the opening of the structure.
A commonly-owned, copending application, Ser. No. 08/754,916, filed Nov. 22, 1996, whose contents, including the contents of the patents incorporated by reference in the said copending application and the contents of a continuation-in-part application, Ser. No. 08/885,095, are herein incorporated by reference, describes apparatus for generating uniform magnetic fields in a region of interest employing wedge-shaped blocks as the primary generator of the main magnetic field. In that magnetic structure, there is no necessary relationship between the orientation of the magnetic field in a cavity defined by the wedge-shaped blocks and the block surfaces interfacing with the cavity. A major thrust of this application is how to generate a field that is sufficiently uniform for use in MRI by means of a relatively open magnetic structure allowing surgical intervention during an MRI procedure.
While the magnetic structures described in these referenced patents and applications are suitable for their intended purpose, nevertheless a possible shortcoming is the generation of a substantial fringe field in the medium surrounding the magnetic structure as a result of opening up the structure for surgical intervention. The fringe field can undesirably interfere with a surgeon's use within or adjacent the magnetic structure of various instruments and equipment which may be of ferromagnetic material and may be subject to magnetic field forces.